.img_div {
    /*margin: 20px 400px 0 400px;*/
    position: relative;
    width: 100px;
    height: 100px;
}
.mask {
    position: absolute;
    top: 0;
    left: 0;
    width: 100px;
    height: 100px;
    background: rgba(101, 101, 101, 0.6);
    color: #ffffff;
    opacity: 0;
}
.mask h3 {
    text-align: center;
}
.img_div a:hover .mask {
    opacity: 1;
}